Commercially, parts to be plated can be plated on a continuous basis on reel-to-reel selective plating machines, see "Continuous Reel-to-Reel Plating for the Electronics Industry" by Jean Lochet et al, an AES Electronics Lecture. Such machines are very expensive and perform all the plating steps on a continuous basis, including cleaning, activation, undercoating, and final plating of the parts by processing the parts, in successive steps, through the complete plating cycle. Basically, their processing speed is only limited by the deposition speed, i.e. the ability of the plating baths to produce acceptable deposits of required thicknesses rapidly. It can be seen as a matter of economics that high deposition rates are highly desirable, since the higher the production is, the lower the unit cost becomes.
As a general rule, higher gold concentrations permit higher efficiency, current densities and plating rates. However, for economic reasons (lower inventory, lower drag out, etc.) gold contents should be kept as low as possible.
It will be understood that higher current densities mean higher rates of deposition, since theoretically one ampere will deposit a definite amount of metal in one second. A further consideration is that the current efficiency, expressed as mg/ampere-minute, when reduced to very small values, renders the buildup of thick bright deposits difficult or impossible in high speed applications in which thick deposits have to be built up in a very short time, termed "retention time". That is, the low current efficiency works oppositely to the effect of high current density. Further, as stated in U.S. Pat. No. 4,436,595 at column 3, lines 25-29, the lower the temperature, the brighter the deposit, but the slower the plating speed, and vice versa; and as a compromise between brightness and plating speed, an operating temperature of 130.degree. F. is preferred. In fact, in practice, very few if any known acid gold plating baths give bright deposits at 150.degree. F., whereas, as will be seen in the ensuing description, the reverse is true for the baths of the present invention.
It is an object of the invention to achieve improvements in the high speed plating of gold including increased current efficiency and lower content of cobalt or nickel hardeners in the plated deposit.
The use of nickel or cobalt chelates as brightener/hardeners is taught in U.S. Pat. Nos. 3,149,057 and 3,149,058. The use of aliphatic acids of 2 to 8 carbon atoms such as acetic, citric, tartaric, etc., when properly neutralized to act as buffers to maintain a pH between 3 and 5, is described.
U.S. Pat. No. 3,929,595 which is directed to employing a heterocyclic azohydrocarbon sulfonic acid or salt current extender, and a reduced amount of non-noble metal additions, also discloses the use of a weak organic acid, preferably citric or tartaric acid. It mentions that additional suitable weak organic acids include formic acid, lactic acid, kojic acid, itaconic acid, citraconic acid, gluconic acid, glutaric acid, glycolic acid, acetic acid and propionic acid.
U.S. Pat. Nos. 3,893,896 and 4,075,065 disclose alkali metal gold cyanide plating baths containing a metallic hardener such as cobalt citrate and nickel sufamate, a Lewis acid such as boric acid, zirconium oxychloride and vanadyl sulphate, and a weak, stable aliphatic acid containing one or more carboxylic acid or hydroxy groups. It discloses as suitable organic acids, itaconic, citraconic, gluconic, glutaric, glycolic, citric, kojic, malic, succinic, lactic, tartaric and mixtures thereof.
U.S. Pat. No. 4,615,774 discloses a citrate-free bath for the electrodeposition of a gold alloy, which bath consists essentially of a bath soluble source of gold in an amount to provide a gold content of 4 to 50 g/l, a bath soluble source of nickel alloying metal in an amount to provide a nickel content of 0.5 g/l, oxalic acid in an amount of 20 to 100 g/l, and formic acid in an amount of 20 to 100 ml/l.
Also of interest is U.S. Ser. No. 912,171 filed Sept. 25, 1986, of which this application is a continuation-in-part, incorporated herein by reference, which discloses a gold plating bath comprising an aqueous solution containing a soluble gold cyanide compound, a water soluble organophosphorous chelating agent, formic acid in a specified concentration, cobalt or nickel which may be introduced as their salts or chelates as brightener/hardeners and sufficient alkali to bring the pH to within a specified range.
As described therein, the plating may be accomplished by any of the commercial means available such as barrel, rack and strip plating equipment and high speed continuous selective plating equipment. The products are useful for industrial purposes, especially for making electrical connections, e.g. as connectors. Depending on the type of equipment used, plating may be carried out at temperatures in the range of 90.degree. to 160.degree. F. and at current densities from about 0.5 to in excess of 1000 ASF.